evolutionary theory and extraterrestrial life

Although earlier naturalists had realized that living things must have changed
during the history of life on Earth, it was Jean Baptiste de Lamarck
who first developed an evolutionary philosophy based on the key idea of
adaptation. His theory of the "inheritance
of acquired characteristics" remained popular in his home nation for most
of the 19th century and influenced the speculations of Flammarion
and Rosny concerning extraterrestrial life.
Elsewhere, however, Lamarckism was called into question, undermined by the
experiments of Weismann in Germany, and
superseded by Darwin's theory of evolution
by natural selection. According to Darwinism,
the traits favored by natural selection vary in different environments and
cause the genetic change in populations of organisms over generations. This
view had an especially powerful effect on scientific thinking in Victorian
Britain, where it was championed by Thomas Huxley
and the sociologist Herbert Spencer, and
used by H. G. Wells as the basis for his essays
and stories about possible alien life-forms. In time, Darwinian ideas would
transform and unify biology, helping it to become a modern, autonomous science,
but they could not in themselves make biology into a universal science akin
to Newtonian physics. Whereas the movements of the Moon and planets could
be readily observed and therefore used as the basis for a generalization
of the theory of gravity beyond the Earth, no data on extraterrestrial organisms
were available from which the biologist could establish some cosmic principles
of life. Consequently, there was no way of predicting if, on other Earthlike
worlds, life would appear as a matter of course and, if it did, how far
its development would progress and in what directions.

For the most part, biologists in the first half of the 20th century steered
clear of questions about extraterrestrial life, having problems enough in
trying to understand how life had started here (see life,
origin). The tendency among those biologists, such as W. D. Matthew,
who did venture an opinion, was to argue that since life appeared to have
originated on Earth only once or at most a few times over the course of
many millions of years, and then as the result of an incredibly unlikely
chain of chemical reactions, the chance of it happening elsewhere was very
low and perhaps even zero. Astronomers, on the other hand, except during
the 1920s and 1930s when the catastrophic
hypothesis was resurgent, tended to assume that the vast number of stars
and presumed accompanying planets virtually guaranteed that life was abundant,
whatever the details of its origin. Greater interest and urgency was added
to this debate in the 1950s by the results of laboratory work in prebiotic
synthesis, including the Miller-Urey Experiment,
and the imminent start of planetary exploration and proposals to search
for life on Mars and other neighboring worlds.
The laboratory creation of the basic chemical building blocks of life and
also of structures that resembled cell membranes
encouraged researchers involved in origin of life studies and in the nascent
field of astrobiology to express optimism
that the emergence and evolution of life is essentially deterministic –
an inevitable consequence given certain starting conditions. This optimism
was dented somewhat by the failure of the Viking
spacecraft to find incontrovertible evidence of life on Mars and, more seriously,
by the failure to synthesize in the lab more complex prebiotic chemicals,
including proteins and nucleic
acids. Jacques Monod, Francis Crick,
Fred Hoyle, Norman Horowitz,
and Ernst Mayr were among those who spoke out against the consensus view
of astrobiologists. But more recently, the pendulum has swung again in favor
of the idea that a "life principle" amy operate in the universe tending
to produce increasingly complex organic systems from simple beginnings wherever
conditions remotely allow it. This is one of the more spectacular suggestions
arising from complexity theory.

There was also, and continues to be, disagreement over how different extraterrestrial
life is likely to be from the varieties found on Earth (see extraterrestrial
life, variety). Again, the dispute centers on the relative importance
attributed to factors of chance and "contingency" on the one hand, and necessity
and determinism on the other. How far up the ladder of life do significant
differences begin to appear between the life-forms that have evolved on
one world and those that have evolved elsewhere? The commonality in space
of basic organic chemicals such as amino acids
suggests that they and the larger units into which they polymerize, including
proteins, may be general ingredients of life throughout the Universe. The
phenomenon of convergence also argues
in favor of cosmic similarities between life-forms. Set against this, however,
is the vast morphological diversity of organisms on Earth and the major,
unpredictable effects that random cosmic events such as asteroid impacts
can have on the course of evolution (see cosmic
collisions, biological effects). These factors suggest that, over all
of space and time, life must be almost unimaginably diverse.